1. Field of the Invention
The present invention relates to a method and an apparatus for Inverse Fast Fourier Transform (IFFT) in a communication system. More particularly, the present invention relates to a method and an apparatus for adaptively changing an IFFT size according to a size of information to transmit at a transmitter in an Orthogonal Frequency Division Multiplexing (OFDM) or a Single Carrier Frequency Division Multiple Access (SC-FDMA) communication system.
2. Description of the Related Art
In response to increasing demands for not only voice communications but also data services, such as various multimedia Internet services in a wireless communication market, an Orthogonal Frequency Division Multiplexing (OFDM) scheme and a Single Carrier Frequency Division Multiple Access (SC-FDMA) scheme are attracting attention as wireless transmission techniques to meet those demands. The OFDM scheme is adopted by Institute of Electrical and Electronics Engineers (IEEE) 802.16e WiBro or mobile WiBro standards, wireless Local Area Network (LAN), and 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). The SC-FDMA scheme is used to raise a power efficiency of a terminal by lowering a Peak-to-Average Power Ratio (PAPR) which is a shortcoming of the OFDM scheme, and is currently adopted as an uplink scheme in the 3GPP LTE.
The OFDM scheme raises a data rate by converting an input serial data stream into N-ary parallel data streams and transmitting the data on separate subcarriers. FIG. 1 is a block diagram of a conventional transmitter and receiver in the OFDMA system. Referring to FIG. 1, a transmitter of the OFDM system allocates a transmit symbol XK to transmit into an intended frequency band through a subcarrier mapping 101, performs an N-point Inverse Fast Fourier Transform (IFFT) 103, inserts a Cyclic Prefix (CP) into the IFFT-processed transmit symbol and converts the IFFT-processed transmit symbol into serial data 105, applies digital/analog conversion and Radio Frequency (RF) processing 107 to the data, and then transmits the signal via a channel to a receiver. Referring to FIG. 1, the receiver of the OFDM system receives the signal from the transmitter, applies RF processing and analog/digital conversion 111 to the received signal, eliminates the CP 113, performs N-point FFT 115, performs subcarrier demapping and equalization 117, and detects and restores the transmit symbol 119.
In contrast, unlike the OFDM, a signal to transmit is defined in the time domain according to the SC-FDMA. FIG. 2 is a block diagram of a conventional transmitter and receiver in the SC-FDMA system. Referring to FIG. 2, a transmitter of the SC-FDMA system converts the time-domain transmit symbols XK into a signal of the frequency domain through an M-point Discrete Fourier Transform (DFT) 201, allocates the transmit symbols to an intended frequency band through a subcarrier mapping 203, and performs an N-point IFFT 205. Next, the SC-FDMA transmitter adds a CP to the IFFT-processed transmit symbol and converts the IFFT-processed transmit symbol into serial data 207, applies the digital/analog conversion and the RF processing 209, and transmits the signal via a channel to the receiver, as in the OFDM. Referring to FIG. 2, the receiver of the SC-FDMA system receives the signal from the transmitter, applies RF processing and analog/digital conversion 211 to the received signal, eliminates the CP 213, performs the N-point FFT 215, performs the subcarrier demapping and the equalization 217, converts the frequency-domain signal into the time domain signal through M-point Inverse DFT (IDFT) 219, and detects and restores the transmit symbol 221.
As discussed above, the conventional OFDM or SC-FDMA transmitter performs the IFFT in a size corresponding to an entire system band. In more detail, the conventional transmitter carries out the IFFT in the size corresponding to the whole system band, regardless of the information to be transmitted. Even when the size of the information to be transmitted is small, the IFFT is performed based on the size corresponding to the entire system band. As a result, many computations are performed, thereby resulting in unnecessary power consumption. The unnecessary power consumption shortens the amount of time a terminal is able to communicate.